The invention relates to a liquid crystal display device comprising, between a polarizer and an analyzer, a first layer of twisted liquid crystal material having a twisted structure between two transparent substrates, and a second layer of twisted liquid crystal material having a twisted structure between two transparent substrates, with a twist sense which is opposite to that of the first layer of twisted liquid crystal material and with a substantially identical twist angle.
Such double cells are often used in, for example, automotive displays. The two cells may have, for example, a common substrate of, for example, glass or a synthetic material. The used liquid crystal effect is mostly the (super)twisted nematic effect (S)TN, but also other liquid crystal materials such as discotic materials are applicable.
An example of such a double cell is shown in, for example, U.S. Pat. No. 5,287,207. Notably for S-TN double cells, the second cell usually functions as a color compensator. Although a satisfactory contrast is obtained in the case of perpendicular passage of the light, and also for, for example, orange light in the case of oblique passage, it appears to be difficult to manufacture such a double cell in a simple manner (for example, by adapting the thickness(es) and/or birefringence of the layer(s)) with a satisfactory contrast upon oblique passage of, for example, green light or white light for which there is a great demand in displays for xe2x80x9cautomotivexe2x80x9d applications. The contrast is considerably lower for green light at an angle, while leakage of light through non-addressed pixels occurs.
It is, inter alia, an object of the invention to obviate one or more of the above-mentioned problems. To this end, a liquid crystal display device according to the invention is characterized in that at least one optical retardation foil is present between the analyzer and the polarizer, which retardation foil has an optical principal axis which is substantially parallel to one of the optical axes of the analyzer (transmission or absorption axis).
A retardation foil is understood to be a self-supporting or non-self-supporting layer of a birefringent material, or a layer having an optically compensating or retarding effect (an optically anisotropic layer). Upon birefringence, the refractive index varies in dependence upon the direction of the vector of the electric field associated with a light ray. Birefringent material has only one axis for which a light ray with the vector of the electric field along this axis is diffracted at an extraordinary refractive index ne. The relevant axis is referred to as the optical principal axis of the material. For light rays with the vector of the electric field perpendicular to this axis, the refractive index may be the same for all directions (ordinary refractive index no). If the refractive index varies perpendicular to this axis, then a bi-axial material is concerned. In this case, one of the axes, preferably the axis having the largest refractive index, is parallel to the (transmission) axis of the analyzer.
Said retardation foil may comprise a plurality of retardation layers which are jointly present in one position (for example, between the double cell and the analyzer), but these (sub-)layers may be alternatively present at separate locations.
It is found that the contrast is considerably enhanced due to said choice of the retardation foil with a principal axis which is substantially parallel to the transmission axis of the analyzer (deviation of at most xc2x15%), notably when a value R of between 100 and 400 nm is chosen for the retardation of the foil R (or of the joint (sub-)layers). The invention is notably applicable to STN double cells having a positive optical anisotropy (xcex94n=(nexe2x88x92no) greater than 0). The invention is also applicable to double cells in which the second cell (referred to as compensator cell) comprises a twisted liquid crystal material having a negative optical anisotropy, for example, discotic materials. The twist angles are then preferably in a range between 45 and 315 degrees.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.